There are currently two basic types of focusing devices for use with a photographic enlarger known in the prior art, to gain a complete understanding of any focusing device, a basic familiarity with the photographic enlarging process is a prerequisite. The basic elements of the photographic enlarger which functionally cooperate with the focusing device are the light source, the lens and the easel. These three elements are arranged in a stacked vertical arrangement and a photographic negative is placed between the light source and the lens. The image of the photographic negative is projected onto the surface of the easel. Since the printing paper is of negligible thickness and is placed on the surface of the easel, the image projected onto the easel surface will be substantially and for all practical purposes the same image as is ultimately transferred onto the print paper. Therefore, throughout the disclosure the easel surface is referred to as a fixed measuring point for convenience of description but it is to be understood that the surface of the print paper is technically the precise distance measuring point. Therefore, to achieve ultimate accuracy, it is possible to place a sheet of material on the easel surface which is the same thickness as the printing paper which will eventually be used and then make all measurements from the surface of that sheet of material. The lens is adjustable to vary the distance between it and the photographic negative to thereby properly focus the projected image of the negative on the surface of the easel. To aid the photographer in determining the proper adjustment of the lens with respect to the photographic negative, the focusing devices of the prior art were developed. The principal of operation of the focusing device was to reflect the projected light from the enlarger's light source with a mirror mounted in the focuser onto a viewing surface portion of the focuser. To obtain the same image in the plane of the viewing surface of the focuser as would appear at the surface of the easel, the distance from the midpoint of the mirror to the plane must be exactly equal to the distance from the midpoint of the mirror to the easel. Therefore, the distance that a light ray would travel from the lens of the enlarging device to the surface of the easel is the same as the distance a light ray will travel from the lens of the enlarging device to the midpoint of the mirror plus the distance from the midpoint of the mirror to the surface of the viewing plane. Some prior art focusers, such as McRae U.S. Pat. No. 2,082,170, profess the ability to properly operate at any point on the easel surface but based on well-established laws of optics, their structure precludes such operation. For very minor displacements from the center point, the angular placement of the mirror surface as compared to the viewing surface will correct for any slight distance deviation and a correct image will be maintained. However, when the prior art focuser is moved substantially off center then the above distance equation becomes unbalanced and the angular positioning of the mirror and viewing surfaces is inadequate to correct the discrepancy. In this latter case, the prior art focuser will produce at best a false image causing improper adjustment to the position of the enlarging lens and more likely will produce no image at all.
The first type of the prior art focusers uses a finely ground glass surface as the plane upon which the reflected image of the photographic negative is focused. To improve the accuracy of this focusing device, a simple magnifying lens is commonly positioned above the ground glass surface. The second type of focuser known in the prior art works essentially in the same manner as the ground glass focuser just described. The primary difference being that the ground glass screen is replaced by a high-powered optical eye piece which focuses on an aerial image of the projected negative. At the plane where the reflected image is focused, a clear glass reticle is positioned. A pattern is commonly etched on the reticle so that the eye piece can be focused to suit the individual photographer. By the use of the high-powered eye piece, a greatly enlarged view of the image on the easel is produced. Other than the increased power of magnification of the reflected image, both the ground surface-type device and the clear glss reticle-type device operate in the same manner.
Both prior art type focusers have fixed bases which are designed to sit on the easel surface in the center of the projected image. These focusing devices have a fixed reflecting mirror which is positioned at a small acute angle from the horizontal. Therefore these focusing devices, for proper operation, must be placed only in the center of the projected image on the enlarging easel and the focus of the image at the corners or any position other than the center of the projected image is not possible. When the focusing devices of the prior art are placed in any position other than the center of the projected image on the easel, a false image is seen at the viewing surface since the measurement of the distance that the light ray will travel from the enlarging lens to the surface of the easel is not equal to the distance that a light ray will travel from the enlarging lens to the midpoint of the mirror surface plus the distance from the midpoint of the mirror surface to the projected image viewing point. Therefore, it is not possible to measure the proper focusing for the projected image at any point other than the center of the projected image itself since only at this point are the above distances equal. Another disadvantage of the prior art occurs when the easel itself is tilted to compensate for vertical distortion such as may be present when tall buildings are positioned inward towards the center of the picture. The prior art focusers based upon the above explanation of their operation can not operate properly under these circumstances.